1. Field of the Invention
The present invention relates to motor drive systems, and more particularly to a technique for design/control of a motor drive system including a DC-AC inverter and a synchronous motor.
2. Description of the Related Art
Large vehicles such as aircraft and ships include a multitude of electrical and mechanical systems that perform complex tasks and require large amounts of energy. Synchronous motor drive systems are suitable for large vehicles. Synchronous motor drives are energy conversion systems that can produce large amounts of energy at high power and high speed, as needed for complex equipment onboard large vehicles. Synchronous motor drive systems are especially efficient with the use of permanent magnet synchronous motor (PMSM) systems due to reduced resulting size and weight.
A typical motor drive system topology for advanced system environments such as “more electric” aircraft with “fly-by-wire” systems is a three phase voltage source DC-AC inverter-driven synchronous motor. The inverter could be installed remotely from the motor, which is integrated into a system such as a cabin air compressor. When the inverter is not in close proximity to the motor, an output AC differential LC filter is used to mitigate high voltage variations occurring on the motor end. Such high voltage variations correspond to high voltage-time derivatives and are caused by long cables connecting the motor to the inverter. The output AC differential LC filter reduces the stress on the motor and helps meet electromagnetic compatibility requirements.
A motor drive system usually needs to accommodate various input DC bus voltage ranges, which ultimately cause variations in the limits of the inverter output AC voltage range. A motor drive system also needs to accommodate various load conditions. The variability in inverter output AC voltage range and load conditions impacts motor drive system performance.
Commercial motor drive systems are generally designed for constant torque operation below the rated operation speed, and constant power operation above the rated operation speed. The rated operating point for the motor drive system is located at the rated speed, where maximum power is achieved. Hence, the inverter and the motor are optimized separately based on the rated operation point.
When high power from a motor drive system is required, the DC bus voltage applied to the motor drive system needs to be increased accordingly, to accommodate the increased back electromotive force resulting from the motor design. The motor current rating is limited by the motor design, which includes the design of motor winding. The winding included in a motor drive system can be large and heavy. Installing, reinstalling, or replacing the winding inside the motor is a difficult task. The DC bus voltage is used to compensate for motor design limitations. For example, medium and high voltage DC buses are designed for megawatts motor drive systems used in commercial applications.
However, in many applications such as the more electric aircraft, the DC bus voltage that can be used to drive high power motor drive systems is limited. In these applications, separate design optimization of motor and inverter typically results in over-designing of the inverter and the motor to accommodate the peak power/peak current requirements. An additional drawback of separate optimization of motor and inverter design is that the motor drive system design is not optimized for overall system efficiency/power density, especially when an output LC filter is installed between the inverter and the motor. Such a filter is often required in applications such as the more electric aircraft, etc.
A few publications have studied improved efficiency schemes for motor drive systems. One such technique is described in “Method and System for Improving Efficiency of Rotating, Synchronous, Electrical Machine Interacting with Power Converter”, Roman Bida, US Patent Application 2002/0149336 A1. With the method described in this work, the efficiency of a motor drive is improved by introducing a spectrum of harmonic components in a power converter supplying energy to the motor. The harmonic components control the current of the power converter so that the current becomes identical in shape and phase to the back electromotive force (back EMF) of the motor. This concept, however, does not provide overall optimization of the motor drive system, as it optimizes only the motor section of the system.
Another technique is described in “Back EMF Controlled Permanent Magnet Motor”, D. Fulton and W. Curtiss, U.S. Pat. No. 4,275,343. In this publication, however, only the back EMF control of the motor is optimized. Again, no overall optimization of the motor drive system for design and control of both inverter and motor is performed.
A disclosed embodiment of the application addresses these and other issues by utilizing a high power density/limited DC link voltage motor drive system, with design and control optimization achieved for the combination of inverter and motor system.